Methyl methacrylate (MMA) is an important chemical used as a starting material in the production of various products, including, among other things, acrylic plastics. One cost-efficient method for producing MMA starts with converting ethylene to propionaldehyde, followed by condensation to form methacrolein (MAL), and subsequent oxidative esterification to form MMA, with water produced as a by-product during oxidative esterification. For the oxidative esterification, the MAL is reacted with molecular oxygen in an alcohol (such as methanol) typically in the presence of a Pd-containing catalyst.
The water produced in the reactor during the oxidative esterification of methacrolein to MMA is believed to have a detrimental effect on conversion and selectivity by interacting with the Pd-containing catalyst used in the oxidative esterification reaction, as indicated in U.S. Pat. No. 6,107,515. Some of the water produced competes with the methanol to react with the MAL, thus affecting the selectivity. Also, water has a tendency to get adsorbed on the active site of the catalyst, thereby reducing the reaction rate as the concentration of water increases (reduced conversion). Recent studies on porous styrene-divinylbenzene copolymer catalyst supports (hydrophobic material with large surface area) for MAL oxidative esterification also suggests that water removal can be critical for the oxidative esterification reactor performance. (See, Wang, B., et al., Journal of Molecular Catalysis A: Chemical, 2013, 379(0), pp. 322-326; and Wang B. et al., Ind. Eng. Chem. Res., 2012, 51(10), pp. 3932-3938.)
To remove water from the oxidative esterification reaction, published methods, such as that described in U.S. Pat. No. 6,107,515 use membranes to continuously remove water from the reaction system. However, membranes are not practical due to low separation fluxes (because of low feed concentrations and temperatures) and extremely high membrane areas that would be required. The continuous removal of water by hydrophilic membranes is thus best suited for small batch reactors and not for commercial scale.
It is desirable to develop a process for the commercial production of MMA by the oxidative esterification of MAL in which water is continuously removed from the reaction in situ.